The terms “Terahertz” and “far infrared” (FIR) identify the range of the electromagnetic spectrum with free space wavelengths of about 10 to 1000 microns, or with wavenumbers from about 1000 to 10 cm−1. Humans have developed extensive technology to generate and detect electromagnetic waves or vibrations throughout the electromagnetic spectrum—from the very short wavelengths and very high frequencies of gamma rays to the very long wavelengths and very low frequencies of radio waves—with the exception of the far infrared (“FIR”) gap in the spectrum existing between infrared light and millimeter wavelength microwaves. In the FIR gap, various sources and detectors exist; but they are not practical (e.g., they lack intensity, frequency-tuning ability and/or stability).
In the late 1980's, the research of the late Professor John Walsh at Dartmouth College and others led to the development of radiation sources that produce electromagnetic radiation at FIR frequencies in a tunable fashion. See, e.g., U.S. Pat. No. 5,263,043 and U.S. Pat. No. 5,790,585, each of which is hereby incorporated by reference. Walsh's work showed that a small, compact and relatively inexpensive table top free electron laser could be a commercially practiced device to generate FIR electromagnetic waves.
The Smith-Purcell (S-P) effect, first observed in 1953, can be seen as the scattering of an electron's evanescent wake field from a grating. The wavelength (λ=2πc/ω) of the emitted radiation is dependent on the grating period (l), electron velocity (ν), and emission angle relative to the beam direction (θ), by the so called S-P relation
      λ    =                  l        m            ⁢              (                              c            ν                    -                      cos            ⁢                                                  ⁢            θ                          )              ,(Equation 1) where m is the diffraction order of the emission. This relation has been confirmed for spontaneous S-P radiation experiments spanning the visible, THz, to microwave spectrum.